1. Field of the Invention
This invention relates to single crystal optical fibers and in particular relates to claddings for, and apparatus and methods for cladding, single crystal fibers enabling low-loss single mode operation, and to devices incorporating clad, single crystal optical fibers.
2. Background of the Invention
The need for cladding optical fibers is well known. Cladding materials reduce optical losses and other deleterious effects in commercial glass and other vitreous fibers. The cladding surrounds the glass fiber core, the portion which guides propagating light waves, with a material having a slightly lower index of refraction than the core. The cladding isolates the core from the outside world and reduces scattering losses along the fiber length.
Glass fibers are generally produced in a pulling process. In this process, a relatively large glass preform, comprising a cylinder of core glass surrounded by the lower index cladding glass, is melted and pulled into a very fine, glass-clad fiber. The process creates a smooth core/cladding interface, insuring very low losses for light traveling through the core. These clad glass fibers are used routinely in transcontinental communications because of their extremely low losses and because the core size can be made so small that only a single mode of light may traverse its length.
Materials other than glass, however, are also being considered for optical fibers. In particular, single crystal optical fibers, meaning fibers grown from a single crystal and having definite crystal planes, show attractive properties which distinguish them from conventional glass fibers. For instance, neodymium YAG (yttrium aluminum garnet)(Nd:YAG) crystals can be formed into both rods and thin fibers. In both forms, the crystal material can amplify light and function as a laser. Lithium niobate crystals exhibit strong electro- and acousto-optical properties. With a proper cladding, such crystals would be useful in fiberoptic devices such as phase and amplitude modulators, acousto-optic modulators, Bragg reflectors, frequency shifters, second harmonic generators, parametric amplifiers and oscillators. In such single crystal fiber devices, though, a lack of suitable cladding impairs their operation.
A low-index cladding for crystal fibers would allow fewer light modes to be guided and would greatly reduce scattering losses. Fiber lasers without cladding require very high pump energies; electro and acousto-optic fiber devices lose much of their light before they can alter it. Single crystal fibers, being grown rather than "pulled", exhibit a relatively large degree of surface irregularities. These irregularities are primarily due to diameter variations along the fiber length, are not found in glass fibers and contribute heavily to the crystal fiber losses.
Single crystal fibers are grown using a laser-heated, crucibleless, pedestal growth apparatus. In such an apparatus, the upper end of a source rod of the crystal material, e.g., Nd:YAG or Lithium niobate, is heated with a focused laser beam. Once the laser beam melts the upper end of the source rod, the lower end of a crystallographically oriented seed rod is dipped into the molten material. The single crystal fiber is then grown by raising the lower source rod at a rate which maintains its upper end in the beam of the heating laser while simultaneously raising the upper seed rod at an even faster rate, drawing the single crystal fiber from the molten material.
Growing such a single crystal fiber differs from the viscous drawing of a glass fiber since, unlike glass, a melting or growing rod of crystalline material has a definite location along its length at which a liquid-to-solid phase transition occurs. There are two liquid/solid interfaces present during growth of a single crystal fiber, one interface at the end of the source rod and another along the growing fiber. Between these two interfaces, there exists a region of truly molten material supported by the surface tension of the liquid material. As the single crystal fiber grows, minute variations in the growth parameters, such as the translation rates and the liquid temperature, cause the diameter of the growing fiber to vary. These diameter variations cause the surface of the single crystal fiber to be rough.
Lithium niobate fibers made by these methods exhibit a rough surface attributable in part to diameter variations, and in part to condensation of gaseous material upon the fiber surface during growth. Such irregularities cause further undesirable scattering of light in the fibers.
Because single crystal fibers are grown from seed crystals and not pulled from a vitreous melt, they cannot be clad in the same way as amorphous glass fibers. One research group attempted to clad Nd:YAG fibers by coating a fiber with a glass frit, which is a layer of ground glass applied like a paint and then baked. The result was an uneven cladding that was inflexible and functioned poorly. We are not aware that anyone has disclosed a suitable cladding for lithium niobate.
Simple, reliable claddings, and methods for making such claddings for grown single crystal fibers would permit the manufacture of many new crystal fiberoptic devices, useful in a variety of laboratory and commercial settings.